The present invention relates generally to paper making and, more particularly, relates to the construction of a press section of a paper machine.
The present invention is particularly directed to improvements in press sections which include, in the direction of web passage through the press section, at least three successive, separate press nips, i.e., press nips formed by their own respective pairs of press rolls, and wherein water is removed from the web to a substantial extent. Each of the press nips has a pair of fabrics passing therethrough so that the web is situated between two fabrics as it passes through each of the three press nips. Dewatering takes place through both faces of the web in at least the first one of the three press nips, the first press nip being preferably formed between two open-faced press rolls whereas the second and third press nips are formed between an open-faced press roll and one of an open-faced press roll and a smooth-faced press roll, and wherein the first upper or lower fabric in the press section acts as a pick-up fabric onto which the web is transferred from the forming wire of the paper machine forming section.
Dewatering of a paper web by evaporation consumes large amounts of energy and is therefore a costly and uneconomical operation. For this reason, it is usually attempted to remove as much water as possible from the web by mechanical means before the web reaches the drying section of the paper machine. The last stage where water is mechanically removed from the web in a paper machine is the press section where water is removed from the web by pressing the web between rolls. It is well known that water will be more readily removed from a web when the temperature of the water is elevated since the viscosity of the water and the coefficient of compression elasticity of the web are thereby reduced as is the surface tension. In this manner, the dry matter or solid content of the web after the press section is remarkably increased resulting in significant economies in the consumption of energy in the drying section of the paper making machine.
As the rates of production of paper machines increase, one of the major bottle-necks which limit further increased production rates are the existence of free draws of the web after the press section, either from the press section to the drying section or in the first free spaces in the drying section itself.
As discussed below, one of the objectives of the present invention is to provide a method and apparatus by which the running quality or efficiency of operation of a paper machine is improved through the intensification of the dewatering of the web in the press section so that the web leaving the press section is dryer and therefore stronger than has been possible using conventional arrangements.
The most common conventional arrangement for dewatering fibrous webs, in particular paper and cardboard webs, is one wherein the web is passed through a press nip formed between two rolls situated in facing relationship to each other. As is well known, one or two press fabrics are used in such dewatering nips, the fabrics carrying the water drained from the web away from the nip and also functioning to carry the web forwardly after the press nip.
One well known press section which utilizes press nips of the type described above is the "Sym-Press" press section manufactured by the assignee of the instant application, Valmet Oy of Finland. The details of the Sym-Press press section are disclosed in Finnish Announcement Publication No. 50,651. The Sym-Press press section is a compact, so-called fully closed press section in which the paper web coming from the forming wire is passed through a first nip region formed between an open-faced roll and a suction roll, between two felts, water being drained from the paper web through both faces of the web. The press section also includes a smooth-faced central roll provided with at least one doctor device. A second nip region is formed against the smooth-faced roll by means of the suction roll mentioned above. At the second nip region, water is drained from the web through the web face which faces the second roll of the first nip region. Moreover, the press section comprises at least one additional nip region after the second nip region which is formed between the smooth-faced central roll, whose diameter is greater than those of the other press rolls in the press section, and an open-faced roll, a felt passing through the additional nip region. This additional nip region is situated substantially on the opposite side of the central roll relative to the position of the second nip region.
In the conventional press sections discussed above it has been necessary to use one or more press-suction rolls. However, the use of press-suction rolls involves considerable drawbacks, discussed in greater detail below.
For example, the perforations of a press-suction roll may leave a marking in the web which detracts from the appearance of the paper and which may affect its surface properties. Press-suction rolls are expensive and require their own separate drive motors and control systems and, furthermore, are noisy during operation. In particular, the drilling required in the manufacture of press-suction rolls is a difficult and costly procedure. The perforations reduce the strength of the mantle of the roll making it necessary to use special metal alloys in the construction of the rolls which, together with the requirement of relatively large thicknesses for the mantle, results in high material costs.
Moreover, press-suction rolls consume large quantities of air since in addition to the air which passes through the web and the felt, the suction system also receives the air that enters into the suction zone in the holes in the suction mantle during each revolution of the roll. Still further, the provision of water tight seals in the suction box of the suction roll causes difficulties.
As noted above, an important drawback inherent in the operation of suction rolls is that a great deal of noise is created during operation. Thus, a secondary objective of the present invention is to reduce the noise generated during the operation of the paper machine in general and of the press section in particular.
It is often desirable in the case of press-suction rolls to provide for a variable crown. However, as a rule, this is not possible since the mantle of the suction roll is perforated and/or since the interior space within the press-suction roll is occupied by the suction box to an extent such that under the circumstances it is not possible to fit devices for adjustably varying the crown therein.
Prior to the advent of a closed and compact press sections, such as the Sym-Press and Sym-Press II press sections of Valmet Oy, open press sections were commonly used which included several separate nips, i.e., wherein each press nip was formed between its own pair of press rolls. In this connection reference is made by way of example to U.S. Pat. Nos. 3,268,390 (Ely) and 4,219,383 (Valkama). One of the reasons why these open press sections were substantially replaced by fully closed and compact press sections was that a compact press section, e.g. the Sym-Press II press section, requires considerably less space than open press sections. Moreover, the open press sections of the type described in the U.S. patents cited above have certain additional drawbacks. For example, the web tends to become moist again after passing through the press nips and this rewetting has been particularly detrimental between the second and third nips and especially where thin paper qualities are being manufactured. As a rule, after the second nip in the prior art open press sections, the dry matter or solid content of the web has been relatively high. However, when the web is transferred into the third nip carried by the press fabric, water is transferred back into the web. For this reason, in the prior art open press sections, attempts were made to detach the web from the press fabric immediately after the third nip. However, this resulted in the necessity of carrying the web as an open draw into the drying section. As is known, open draws increase the risk of web breakage since the strength of the web is still relatively low due to its water content immediately after the press section.
Furthermore, with respect to the press section disclosed in U.S. Pat. No. 3,268,390, the transfer of the web after the second nip onto the second upper press fabric is insured by means of a suction-pick-up roll and a rewetting of the web occurs during the run of the web after the suction-pick-up roll. Moreover, in the press section disclosed in the '390 patent, the web has an open draw after the third press nip into the drying section.
In the press section discussed in U.S. Pat. No. 4,219,383, the web runs through all of the three successive press nips carried by the same lower fabric functioning as the press fabric in each nip. Accordingly, rewetting of the web occurs between the nips when the water is transferred, after the nips, from the lower fabric back into the web. Such rewetting, however, is not particularly detrimental in the case of thick paper qualities for which the press section disclosed in the '383 patent is relatively well suited.